1. Field of the Invention
The present invention relates to a stacked photovoltaic device having at least two power generating function units.
2. Related Background Art
Photovoltaic devices are devices for converting an incident light energy to an electric energy. Among those, a solar cell is a photovoltaic device for converting sunlight as white light to an electric energy, which enables efficient conversion of light of a wide wavelength region. Therefore, in order to achieve a high conversion efficiency, it is necessary to perform efficient light absorption throughout a wide wavelength region.
As a measure therefor, there is well known a stacked photovoltaic device formed by stacking photovoltaic devices having semiconductor layers of different band gaps as photoactive layers. The stacked photovoltaic device efficiently absorbs and utilizes light in a wide wavelength region by disposing a photovoltaic device using a semiconductor of a relatively large band gap at a light incident side to absorb short-wavelength light having a large energy and disposing a photovoltaic device using a semiconductor of a relatively small band gap under the light incident side device to absorb long-wavelength light having a small energy that has passed through the upper device.
Here, it is necessary to introduce into each photovoltaic device light of a wavelength region suitable for that device. This is because the wavelength region of an incident light that can be utilized by each photovoltaic device is limited by the band gap of a semiconductor used for a photoactive layer of that photovoltaic device. That is, a photon having an energy smaller than a band gap energy is not absorbed by a semiconductor and can not be utilized. Further, although a photon having an energy greater than a band gap energy is absorbed by a semiconductor, the potential energy of an electron which can be provided when exciting the electron is limited to the magnitude of the band gap. Therefore, it is impossible to use a difference component between the band gap energy and the photon energy. That is, in the case of the stacked photovoltaic device, it is important to make only light of a short-wavelength region incident on the light incident side device of a stacked photovoltaic device and to make only light in a long-wavelength region incident on the underlying device.
As a measure therefor, there is known a method of providing a transparent conductive film between upper and lower photovoltaic devices and using the film as a selective reflection layer. For example, Japanese Patent Application Laid-Open No. S63-77167 or “Thin film polycrystalline silicon solar cell” by Kenji Yamamoto, Applied Physics, Japan Society of Applied Physics, Fifth Edition, Volume 71 (May, 2002), pp.524–527 disclose a method of providing a conductive layer as a selective reflection layer for reflecting short-wavelength light and passing long-wavelength light therethrough between photovoltaic devices. Further, Japanese Patent Application Laid-Open No. H02-237172 discloses a method of adjusting the film thickness of the selective reflection layer to conform the peak of the reflectivity of the layer to a maximum wavelength of the spectral sensitivity of a light incident side photovoltaic device, thereby increasing the current value of the light incident side photovoltaic device. Those methods aim at preventing short-wavelength light to be originally absorbed by a light incident side photovoltaic device from being absorbed by an underlying photovoltaic device to thereby improve the conversion efficiency of the light incident side photovoltaic device.
Incidentally, the selective reflection layer needs to have a function of light reflection as well as a function of establishing an electrical connection between plural devices. At this time, the selective reflection layer functions as an external resistor from the viewpoint of an electric circuit. Thus, a large resistance value thereof directly results in a deteriorated fill factor of the device. For that reason, a material having a high conductivity has been hitherto used for the selective reflection layer.
On the other hand, a large-area photovoltaic device such as a solar cell reduces, because of the large area, its conversion efficiency owing to a short-circuit current generated at electrically defective portions of the device resulting from dust or other such foreign matters during film formation. To cope with the reduction is now a big concern. An effective countermeasure against the reduction is, as well known in the art, shunt passivation for immersing the device in an electrolyte, causing a current to flow through the electrolyte, and selectively dissolving a transparent electrode at an electrically defective portion for the removal. This technique realizes the selective removal of the transparent electrode by making use of the fact that the electrically defective portion more easily allows the current to flow therethrough than a normal portion of the device does, and suppresses the generation of the short-circuit current by isolating the electrically defective portion in terms of the electric circuit.
In the conventional photovoltaic device using the selective reflection layer, the selective reflection layer is not an exposed surface layer and thus makes it difficult to exclude the electrically defective portion from the electric circuit through the shunt passivation.
More specifically, as shown in FIG. 8, in a shunt passivation process of the photovoltaic device using the selective reflection layer, a conduction path is formed in a planer form in a selective reflection layer 102 having a low resistance between a first photovoltaic device 101 and a second photovoltaic device 103. Thus, in an electrically defective portion 105 of the second photovoltaic device 103, concentration of a passivation current 106 does not occur and a transparent electrode 107 is only removed at an electrically defective portion 104 in the first photovoltaic device 101.
Further, as shown in FIG. 9, if power generation starts in this state, a short-circuit current 202 flowing through the electrically defective portion 105 is spread in a planer form since a conductive film of the electrically defective portion 105 in the second photovoltaic device 103 is not removed. As a result, carriers disappear at a junction surface 203 between a photocurrent 201 of the second photovoltaic device 103 and the short-circuit current 202, leading to diminished electromotive force.
Further, in the case where the transparent electrode 107 at the electrically defective portion 104 in the first photovoltaic device 101 is not fully removed in the shunt passivation process, the short-circuit current is also spread in a planer form in the first photovoltaic device 101 as mentioned above, which further diminishes the electromotive force.
In particular, the conventional selective reflection layer 102 is made of a material having a high conductivity and shows a lower sheet resistance. As a result, the short-circuit current is too widely spread in a planer form, causing a drop of the conversion efficiency of the whole device.
As mentioned above, up to now, even though the selective reflection layer is incorporated for increasing the photocurrent, the photovoltaic device having less electromotive force can only be produced.